This disclosure relates to an image reading apparatus and an image forming apparatus provided with the image reading apparatus.
An image forming apparatus, such as a copying machine or the like that forms an image on paper using an electrographic method, includes an image reading apparatus configured to optically read a document image. In recent years, an LED is used as a light source for the image reading apparatus in substitution for a conventional xenon lamp or a cold cathode lamp.
An image reading apparatus using an LED as a light source includes a type that uses a side-light method in which light emitted from an LED disposed on one end in the main scanning direction is introduced into the main scanning direction by a light guiding body. FIG. 7 illustrates the configuration of an illumination unit of a conventional image forming apparatus employing a side-light method.
FIG. 7 is a side view of the illumination unit of the image reading apparatus employing a side-light method. As illustrated in FIG. 7, an LED 106 is disposed as a light source on one end of the main scanning direction (the transverse direction in FIG. 7). A cylindrical light guiding body 107 is disposed in proximity to the LED 106 along the main scanning direction. An incident surface 107a of one end in the main scanning direction of the light guiding body 107 faces the LED 106. Prisms 121 that include a plurality of V-shaped grooves on a lower surface are disposed on the opposite side to an emission surface 107b on the upper surface of the light guiding body 107 at a suitable interval in the main scanning direction. A platen glass 123 is provided above the light guiding body 107 to mount the document.
When the LED 106 is illuminated, the light becomes incident from the incident surface 107a of the light guiding body 107 on an inner portion of the light guiding body 107 while being subjected to refraction in accordance with Snell's law as illustrated in FIG. 7, and is completely reflected by the inner surface of the light guiding body 107 and is propagated in the main scanning direction. A part of that light is reflected by the incident surface of the prism 121, is emitted from the emission surface 107b through the platen glass 123, and illuminates a document that is mounted on the platen glass 123.
The adoption of this type of side-light method has the advantage of reducing the number of LEDs 106 that are used as a light source in the image reading apparatus (image forming apparatus). This type of side-light method is advantageous from a cost consideration. In addition, there is the advantage that loss of light energy due to propagation in the inner portion of the light guiding body 107 can be suppressed to a low value while all the light is reflected in accordance with Snell's law.
However, when a LED that emits white light is used as the LED 106, as illustrated in FIG. 8, the LED 106 is configured by disposing a yellow phosphor 106b on a blue LED chip 106a, and pseudo-emitting white light by superimposition of the blue and yellow colors.
However, the luminescence distribution of blue and yellow light are different from each other as illustrated in FIG. 8. The blue-light LED chip 106a has a small illumination surface area relative to the overall illumination surface area of the LED that emits white light (surface area of the yellow phosphor 106b), and the luminescence distribution of blue light is narrow when compared with the luminescence distribution of yellow light. Therefore, the luminescence distribution of blue light after passing from the incident surface 107a of the light guiding body 107 to become incident into the light guiding body 107 is narrow relative to the luminescence distribution of yellow light.
In this context, the white LED light that illuminates the document surface at a position corresponding to a position in proximity to the incident surface 107a of the light guiding body 107 is completely reflected directly by the prism 121 without contacting the side surface of the light guiding body 107. As a result, the luminescence distribution of the white LED light illuminates the document surface in proximity to the incident surface 107a of the light guiding body 107, and in particular, has a considerable effect on the cross-sectional distribution of light. Although yellow light that has a wide luminescence distribution and the cross-sectional distribution of that light is wide, blue light has a narrow luminescence distribution and the cross-sectional distribution of that light is narrow. Therefore, when the document separates from the platen glass, or the reading optical axis varies, there may be a change in the granularity configuration of a solid image or a tint in the scanning image of the document image corresponding to a position in proximity to the incident surface 107a of the light guiding body 107.
In this context, a technique in which a reflecting body 224 that includes a conical cavity 224a that has an apex near to the LED 206 is disposed between the LED 206 and the end surface of the light guiding body 207 as illustrated in FIG. 9. According to the related technique, light emitted from the LED 206 becomes incident upon the light guiding body 207 via the reflective body 224, and thereby enables an increase in the luminescence distribution immediately after the light becomes incident and an increase in the cross-sectional distribution of blue light in proximity to the LED 206.